1. Field of the Invention
This invention relates to improvements in the impeller/rotor component of a dual-entry, high performance centrifugal compressor.
2. Description of the Prior Art
Dual-entry, single exit centrifugal compressors per se are known in the art. See, for instance U.S. Pat. No. 2,438,426 to Whittle; Johnson, D. and Mowill, R. J. "Aegidius Elling--A Norwegian Gas Turbine Pioneer," Norsk Teknisk Museum pub., Oslo 1968 at FIG. 11. The impeller or rotor component of such compressors typically has a hub with a centrally located, full radius portion flanked by portions of decreasing radius extending to either side of the central portion. Rotor blades for accelerating and compressing the working gas are typically mounted on either side of the full radius central portion of the hub, and are configured to provide a generally decreasing height dimension for the twin gas flow paths established between the rotor hub and the surrounding housing. It is also known to arrange the blades in a spiral, helical fashion when viewed in the direction of the axis of rotation of the hub. See Mowill, R. J. and Strom, S., "New Radial Engine Technology from Kongsberg," ASME pub. 83-GT-221, at FIG. 4.
In typical single entry and dual-entry centrifugal compressors, the rotor blades extend along the entire length of the respective flow path, that is, from the respective rotor axial inlet to the exit at the radial extremity of the central full radius hub portion. It is also known in the design of single and dual-entry centrifugal compressors to utilize one or more additional sets of blades which extend along only a part of the gas flow path, that is, from an intermediate point along the flow path to the compressor rotor exit. These partial length blades generally are positioned in alternating relation with the full length blades about the periphery of the rotor and are intended to assume part of the aerodynamic loading in the vicinity of the outer periphery of the rotor.
Problems that can arise in the utilization of current dual-entry rotor designs in applications calling for a pressure ratio (P exit/P inlet) greater than about 5:1, stem from the effect of variations in the velocity profile of the gas exiting the rotor on the efficiency of the diffuser component of the compressor. In particular, the velocity profile in the axial direction can include a pronounced depression coincident with the full radius central portion of the rotor hub.
The axial velocity profile can also be biased to one or the other rotor side due to different ambient conditions (e.g. pressure, temperature) upstream of the respective rotor axial inlet, which can cause a further decrease in diffuser efficiency.
In conventional high performance single and dual-entry rotors of the type using alternating full length and partial length blades, velocity profile variations can also occur in the tangential direction. The velocity profile would tend to exhibit a series of evenly spaced depressions about the rotor periphery coincident with the locations of the partial length impeller vanes. This profile is a consequence of the uneven division of the gas flow on each side of the partial length blades, for an even blade spacing (i.e. equal flow path cross-sectional flow areas).
These variations in the velocity profiles can have the effect of decreased diffuser component efficiency. While the aforementioned variations in the velocity profile of prior art compressor rotors can be expected to adversely affect the performance of all diffuser types, the performance penalties would be especially severe for closely coupled pipe-type diffusers, that is, pipe diffusers positioned closely adjacent the rotor blade tips.